1. Field of the Invention
The present invention relates to an improved metallic tubular cladding for nuclear fuel rods, which will prevent formation of hydride blisters on the cladding and possible resultant failure of the cladding.
2. Description of the Prior Art
In the production of electrical energy from nuclear reactor systems, the reactor provides heat for steam production and subsequent production of electricity. In the nuclear reactor, nuclear fuel rods are present which contain a nuclear fuel such as pellets of uranium dioxide or a mixture of uranium and plutonium dioxides. These fuel rods are metallic tubular shells, or cladding, which contain the fuel pellets and which must maintain their integrity so as to prevent any leakage into or out of the shell. Some of the conventional light water reactor (LWR) fuel rod designs and support structures are described in J. T. A. Roberts, "Structural Materials in Nuclear Power Systems," Plenum Press, New York, (1981), at pp. 53-61. This document is hereby incorporated by reference.
The conventional cladding type, at present, is a cladding of a zirconium-based alloy, such as, Zircaloy-2, Zircaloy-4, and zirconium-2.5 weight percent niobium alloy. Other types of cladding have been proposed. These proposals have included cladding of a zirconium base material with various coatings or barrier means provided on the inside wall to protect the cladding from attack by constituents released from the nuclear fuel during operation of a reactor containing the fuel rod. As examples of such coatings or barrier means that cover the full surface of the cladding, U.S. Pat. No. 4,022,662 described a cladding in connection with a separate unattached metal liner comprised of stainless steel, copper, copper alloys, nickel or nickel alloys, the liner disposed between the cladding and the fuel material. A diffusion barrier of chromium or chromium alloy is also disposed between the cladding and the metal liner. In U.S. Pat. No. 4,045,288, a composite fuel element cladding is described which comprises a zirconium alloy substrate having a metal barrier of 1-4 percent of the wall thickness formed from niobium, aluminum, copper, nickel, stainless steel and iron, and an inner layer of stainless steel, zirconium or a zirconium alloy metallurgically bonded on the inner surface of the metal barrier. U.S. Pat. No. 4,093,756 and U.S. Pat. No. 4,137,131 disclose processes for electroless deposition of a metal film, such as nickel, on a zirconium or zirconium alloy article, and refer to an earlier copending application, Ser. No. 522,769 filed November 11, 1974, which was abandoned in favor of a continuation application Ser. No. 725,824 filed Sept. 23, 1976. That earlier application related to a composite cladding comprising a zirconium-based outer layer and on the inside surface, a layer of copper, nickel, iron or iron alloys.
These, and other patents, have discussed the problems associated with cladding materials and have proposed means for overcoming such problems.
One particular problem that is present in cladding for nuclear fuel rods is the formation of hydride blisters on the cladding inner surface which can lead to eventual fracture or puncturing of the cladding. As discussed in U.S. Pat. No. 4,022,662, referred to above, hydrogen gas can be formed by slow reaction between the cladding, and residual water or other hydrogenous impurities in the sealed rod. Under certain conditions, localized hydriding of the interior surface of the cladding can form a single massive hydride blister which can result in failure of the cladding. Localized hydride blisters form when the zirconium oxide film, which may be intentionally or unintentionally formed on the inner surface of the cladding, becomes scratched or otherwise defective at a very small number of areas (usually one) which acts as a localized hydride sink.
Efforts to prevent hydride blistering have included vacuum outgassing of the fuel rod to remove hydride formatives, which is an expensive and difficult procedure; the use of getters at the top portion of the fuel rod, as described for example in U.S. Pat. No. 4,279,700 assigned to the assignee of the present invention; or extensive drying of the fuel pellets and dry storage thereof to remove all moisture, another expensive and difficult procedure.
An object of the present invention is to provide an economical and efficient fuel rod cladding that is resistant to formation of hydride blisters.